QUANTICO, Va. -- The U.S. military Saturday night identified a tactics instructor from Pacifica as the Marine who shot two fellow Marines to death Thursday before killing himself.

One of those killed by Sgt. Eusebio Lopez, 25, was Lance Cpl. Sara Castromata, a 19-year-old warehouse clerk from Oakley who joined the corps in December 2011, investigators said.

Lopez also killed Cpl. Jacob Wooley, 23, of Guntown, Miss., a field radio operator, inside barracks at the Marine Corps Base Quantico in northern Virginia, investigators said. Other than to say the three Marines worked together at the school, military officials have not described their relationship or released a motive for the shooting.

Lopez was a machine gunner who joined the corps in May 2006 and deployed in support of the wars in Iraq and Afghanistan.

He was an instructor at officer candidates school, known for its grueling 10-week program that evaluates Marines on physical stamina, intelligence and leadership. The candidates must complete obstacle courses, hikes of up to 12 miles in full combat gear and take classes on navigation and tactics that help them in the field, according to the school's website.

Lopez's great-grandfather, also Eusebio Lopez, said the Marines contacted the family Friday night.

"They told us they were investigating more, and they'd let us know. He wasn't the type to do stuff like that," said the elder Lopez, 81.